This invention relates to image-forming apparatus of the type having a substantially finite length duplex path. It also relates to an inverter usable in such a duplex path and in other applications.
U.S. Pat. No. 5,006,900 to Baughman et al, granted Apr. 9, 1991 shows a typical electrophotographic copier/printer in which toner images are formed on a seamed belt image member and transferred to a receiver sheet at a transfer station. To make duplex copies, the receiving sheet is fed through a finite length path in the form of a loop back to the transfer station. In the course of passing through this duplex path, the receiving sheet is turned over at an inverter so that the opposite side of the sheet is presented to a toner image when it returns to the transfer station. The inverter includes a pair of reversing nip rollers which drive the receiving sheet into an inverter guide until they are clear of the entrance of the inverter. The reversing nip rollers are then driven in the opposite direction to drive the edge of the sheet that had been the trailing edge into a pair of exit rollers and on through the duplex path.
The particular apparatus in the Baughman et al patent was designed to work with an image member that had dedicated image frames. Large size sheets took up double-frames on the image member and small size sheets took up one frame with other variations in sheet or image size being absorbed by a variable interframe. Thus, the duplex path length could readily be an integer multiple of the double-frame in-track length to bring the sheet back to the transfer station in good timing with the next image.
U.S. Pat. No. 5,473,419, filed Nov. 8, 1993 to Russel et al and entitled "Image Forming Apparatus having a Duplex Path With an Inverter" points out that having dedicated frames inefficiently uses the image member except for receiving sheets having an in-track length close to the small or large (double) frame in-track distances. Like the Baughman et al patent, this structure utilizes an image member which is a photoconductive belt having a seam. The seam cannot be imaged upon and therefore makes the image member a finite length for spacing images. The Russel et al application suggests that the images be positioned on the belt to provide the most images of a given length between appearances of the seam. Thus, the image member would be utilized most efficiently for its length for every size image being reproduced. No dedicated frames are involved. This creates difficulties in managing the length of the duplex path which for space reasons is preferably as short as possible. The Russel et al application suggests that the effective length of the return path can be varied by adjusting the speed of movement of the receiving sheet in the path, by varying the path itself by moving guides or, preferably, by varying the length of time the receiving sheet is held in the inverter.
U.S. Pat. No. 5,159,395 to Farrell et al, issued Oct. 27, 1992, is one of a large number of references which disclose various duplex scheduling processes. This reference discloses a very commonly used "interleaf mode" in which images for a particular side (back or front) are made until the duplex loop is filled with one skipped cycle or pitch between each print. Once the receiving sheets approach the transfer station from the duplex path, images are alternated between back and front until the end of the mn when some skipped frames are necessary to finish the last set of receiving sheets in the duplex path. This approach has many advantages including feeding the completed sheets evenly to a finisher or output tray. It also provides a skipped frame in the duplex path between images at all times.
U.S. Pat. No. 5,337,135, granted to Malachowski et al on Aug. 9, 1994, uses a variable speed drive to provide spaces between sheets in a duplex path without skipping frames at the transfer and exposure stations.
U.S. Pat. No. 4,568,169, granted to Wada et al, shows an image-forming apparatus having an infinite image member; i.e., a seamless drum, in which a duplex path transport speed is varied according to the size of the sheet to improve efficiency.
U.S. Pat. No. 4,780,745 to Kodama, granted Oct. 25, 1988, suggests that an inverter in a duplex path can receive a slow-moving sheet and substantially speed it up to ultimately shorten the duplex loop.
In moving paper or other receiving sheets through any paper path at relatively high speeds, it is desirable for costs reasons to have as few sets of rollers or other transport devices operating at varying speeds as possible. Further, reliability problems are more likely to occur when a sheet is being slowed down than when it is being speeded up since the slowing down action tends to create a buckle in the sheet.